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Add some comments in rgba_blender_normal()

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In this way we can see how we get the final blending formula which is
equivalent to Rc=(Sc*Sa + Bc*Ba*(1-Sa))/Ra
This commit is contained in:
David Capello 2017-07-18 23:33:54 -03:00
parent b79512d3b4
commit fd408247cf
1 changed files with 9 additions and 6 deletions
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15
src/doc/blend_funcs.cpp
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@ -182,13 +182,16 @@ color_t rgba_blender_normal(color_t backdrop, color_t src, int opacity)
// Ra = Sa + Ba*(1-Sa)
// = Sa + Ba - Ba*Sa
// = Ba + Sa - Ba*Sa
// Rc = Sc*Sa/Ra + Bc*(1-Sa/Ra)
// = Sc*Sa/Ra + Bc - Bc*Sa/Ra
// = Bc + Sc*Sa/Ra - Bc*Sa/Ra
// = Bc + (Sc-Bc)*Sa/Ra
const int Ra = Sa + Ba - MUL_UN8(Ba, Sa, t);
const int Ra = Ba + Sa - MUL_UN8(Ba, Sa, t);
// Ra = Sa + Ba*(1-Sa)
// Ba = (Ra-Sa) / (1-Sa)
// Rc = (Sc*Sa + Bc*Ba*(1-Sa)) / Ra Replacing Ba with (Ra-Sa) / (1-Sa)...
// = (Sc*Sa + Bc*(Ra-Sa)/(1-Sa)*(1-Sa)) / Ra
// = (Sc*Sa + Bc*(Ra-Sa)) / Ra
// = Sc*Sa/Ra + Bc*Ra/Ra - Bc*Sa/Ra
// = Sc*Sa/Ra + Bc - Bc*Sa/Ra
// = Bc + (Sc-Bc)*Sa/Ra
const int Rr = Br + (Sr-Br) * Sa / Ra;
const int Rg = Bg + (Sg-Bg) * Sa / Ra;
const int Rb = Bb + (Sb-Bb) * Sa / Ra;